1. Field of the Invention
This invention relates to a catalytic material for an exhaust gas purifying catalyst, an exhaust gas purifying catalyst for use with an internal combustion engine in which the catalytic material is used, and a method of the exhaust gas purifying catalyst.
2. Description of Related Art
In order for automotive vehicles to purify exhaust gases, it has been known to install in an exhaust line a honeycomb type of exhaust gas purifying catalyst which comprises a honeycomb carrier and a catalyst layer coated on the honeycomb carrier. The exhaust gas purifying catalyst coating layer contains therein ceria (i.e. cerium oxide CeO.sub.2) particles with barium (Ba) fixed to catalytic particles, activated alumina particles and noble metal components. Such a catalyst is known from, for example, Japanese Unexamined Patent Publication No. 3-207446.
The barium (Ba) is fixed to the ceria particles with an effect of preventing thermal degradation of the ceria particles. In order to firmly fix barium to the ceria particles, ceria powders are mixed with a barium nitrate solution and the mixture is dried. A solid lump of barium nitrate adsorbed-ceria is crushed to particles or powders. The barium nitrate adsorbed-ceria powders are mixed with a solution of palladium chloride and water to prepare slurry of exhaust gas purifying catalyst. After having immersed a catalyst carrier in the slurry and formed a catalyst layer containing barium-fixed ceria and palladium on the catalyst carrier. Finally, the catalyst layer is baked and dried.
While the barium contained in the catalyst has a function of preventing thermal degradation of the ceria, it has been known in the art that the barium is actively utilized for purifying exhaust gases and, in particular, for decomposing nitrogen oxides (NOx) in exhaust gases. That is, in the event where the concentration of oxygen in an exhaust gas is high resulting from lean burning, barium (Ba) supported together with catalytic noble metals such as platinum (Pt) by a alumina supporting material or the like is made to absorb nitrogen oxides (NOx) oxidized with the catalytic noble metals so as to temporarily lower the concentration of oxygen in the exhaust gas and consequently create a reducing atmosphere which removes nitrogen oxides from the barium. In the reducing atmosphere, the catalytic noble metals decompose the removed nitrogen oxides (NOx) by utilizing hydrocarbons (HC) and carbon monoxide (CO) as reduction agents in the exhaust gas.
If barium itself contained in a catalyst encounters thermal degradation, the barium is hard to perform the function of preventing thermal degradation of ceria contained in the catalyst and the function of absorbing nitrogen oxides. When barium is exposed to a high temperature of exhaust gas, the barium dissolves and diffuses in and binds to an adjacent supporting material such as alumina particles. In some cases, the barium forms composite compounds (composite oxides) which causes a decline in the primary functions of barium and, in addition, a decline in the activity of catalytic noble metals.
Japanese Unexamined Patent Publication No. 3-2074461 describes another type of an exhaust gas purifying catalyst. That is, slurry of a mixture of a barium nitrate solution mixed with ceria (CeO.sub.2) and boehmite (Al.sub.2 O.sub.3 . H.sub.2 O) and a noble metal solution is applied to a honeycomb carrier. This catalyst coating layer is dried at 130.degree. C and there after baked at 550.degree. C.
Because boehmite includes a portion of alumina Al.sub.2 O.sub.3, barium (Ba) dissolves and diffuses in and firmly fixes to aluminum (Al) when the catalyst coating layer is exposed to a high temperature of heat.